304 research outputs found
UGC 7388: a galaxy with two tidal loops
We present the results of spectroscopic and morphological studies of the
galaxy UGC7388 with the 8.1-m Gemini North telescope. Judging by its observed
characteristics, UGC7388 is a giant late-type spiral galaxy seen almost
edge-on. The main body of the galaxy is surrounded by two faint (\mu(B) ~ 24
and \mu(B) ~ 25.5) extended (~20-30 kpc) loop-like structures. A large-scale
rotation of the brighter loop about the main galaxy has been detected. We
discuss the assumption that the tidal disruption of a relatively massive
companion is observed in the case of UGC7388. A detailed study and modeling of
the observed structure of this unique galaxy can give important information
about the influence of the absorption of massive companions on the galactic
disks and about the structure of the dark halo around UGC7388.Comment: 8 pages, 5 figure
On the global structure of distant galactic disks
Radial and vertical profiles are determined for a sample of 34 edge-on disk
galaxies in the HDFs, selected for their apparent diameter larger than 1.3
arcsec and their unperturbed morphology. The thickness and flatness of their
galactic disks are determined and discussed with regard to evolution with
redshift. We find that sub-L* spiral galaxies with z \sim 1 have a relative
thickness or flatness (characterized by h_z/h the scaleheight to scalelength
ratio) globally similar to those in the local Universe. A slight trend is
however apparent, with the h_z/h flatness ratio larger by a factor of \sim 1.5
in distant galaxies if compared to local samples. In absolute value, the disks
are smaller than in present-day galaxies. About half of the z \sim 1 spiral
disks show a non-exponential surface brightness distribution.Comment: 13 pages, 8 figures, accepted in A and
The origin of polar ring galaxies: evidence for galaxy formation by cold accretion
Polar ring galaxies are flattened stellar systems with an extended ring of
gas and stars rotating in a plane almost perpendicular to the central galaxy.
We show that their formation can occur naturally in a hierarchical universe
where most low mass galaxies are assembled through the accretion of cold gas
infalling along megaparsec scale filamentary structures. Within a large
cosmological hydrodynamical simulation we find a system that closely resembles
the classic polar ring galaxy NGC 4650A. How galaxies acquire their gas is a
major uncertainty in models of galaxy formation and recent theoretical work has
argued that cold accretion plays a major role. This idea is supported by our
numerical simulations and the fact that polar ring galaxies are typically low
mass systems.Comment: 4 pages, 5 figures, stability of the ring discussed, minor changes to
match the accepted version by ApJL. A preprint with high-resolution figures
is available at http://krone.physik.unizh.ch/~andrea/PolarRing/PolarRing.p
Evidence for coupling between the Sagittarius dwarf galaxy and the Milky Way warp
Using recent determinations of the mass and orbit of Sagittarius, I calculate
its orbital angular momentum. From the latest observational data, I also
calculate the angular momentum of the Milky Way's warp. I find that both
angular momenta are directed toward l=270, b=0, and have magnitude 2-8x10^12
M_Sun kpc km s^-1, where the range in both cases reflects uncertainty in the
mass. The coincidence of the angular momenta is suggestive of a coupling
between these systems. Direct gravitational torque of Sgr on the disk is ruled
out as the coupling mechanism. Gravitational torque due to a wake in the halo
and the impulsive deposition of momentum by a passage of Sgr through the disk
are still both viable mechanisms pending better simulations to test their
predictions on the observed Sgr-MW system.Comment: 11 pages, to appear in the February 1 issue of ApJ
SPH Simulations of Counterrotating Disk Formation in Spiral Galaxies
We present the results of Smoothed Particle Hydrodynamics (SPH) simulations
of the formation of a massive counterrotating disk in a spiral galaxy. The
current study revisits and extends (with SPH) previous work carried out with
sticky particle gas dynamics, in which adiabatic gas infall and a retrograde
gas-rich dwarf merger were tested as the two most likely processes for
producing such a counterrotating disk. We report on experiments with a cold
primary similar to our Galaxy, as well as a hot, compact primary modeled after
NGC 4138. We have also conducted numerical experiments with varying amounts of
prograde gas in the primary disk, and an alternative infall model (a spherical
shell with retrograde angular momentum). The structure of the resulting
counterrotating disks is dramatically different with SPH. The disks we produce
are considerably thinner than the primary disks and those produced with sticky
particles. The time-scales for counterrotating disk formation are shorter with
SPH because the gas loses kinetic energy and angular momentum more rapidly.
Spiral structure is evident in most of the disks, but an exponential radial
profile is not a natural byproduct of these processes. The infalling gas shells
that we tested produce counterrotating bulges and rings rather than disks. The
presence of a considerable amount of preexisting prograde gas in the primary
causes, at least in the absence of star formation, a rapid inflow of gas to the
center and a subsequent hole in the counterrotating disk. In general, our SPH
experiments yield stronger evidence to suggest that the accretion of massive
counterrotating disks drives the evolution of the host galaxies towards earlier
(S0/Sa) Hubble types.Comment: To appear in ApJ. 20 pages LaTex 2-column with 3 tables, 23 figures
(GIF) available at this site. Complete gzipped postscript preprint with
embedded figures available from http://tarkus.pha.jhu.edu/~thakar/cr3.html (3
Mb
Polar Ring Galaxies and the Tully Fisher relation: implications for the dark halo shape
We have investigated the Tully-Fisher relation for Polar Ring Galaxies
(PRGs), based on near infrared, optical and HI data available for a sample of
these peculiar objects. The total K-band luminosity, which mainly comes from
the central host galaxy, and the measured HI linewidth at 20% of the peak line
flux density, which traces the potential in the polar plane, place most polar
rings of the sample far from the Tully-Fisher relation defined for spiral
galaxies, with many PRGs showing larger HI linewidths than expected for the
observed K band luminosity. This result is confirmed by a larger sample of
objects, based on B-band data. This observational evidence may be related to
the dark halo shape and orientation in these systems, which we study by
numerical modeling of PRG formation and dynamics: the larger rotation
velocities observed in PRGs can be explained by a flattened polar halo, aligned
with the polar ring.Comment: 22 pages, 8 postscript figures, accepted for publication in
Astrophysical Journa
The Catalog of Edge-on Disk Galaxies from SDSS. Part I: the catalog and the Structural Parameters of Stellar Disks
We present a catalog of true edge-on disk galaxies automatically selected
from the Seventh Data Release (DR7) of the Sloan Digital Sky Survey. A visual
inspection of the , and images of about 15000 galaxies allowed us to
split the initial sample of edge-on galaxy candidates into 4768 (31.8% of the
initial sample) genuine edge-on galaxies, 8350 (55.7%) non-edge-ons, and 1865
(12.5%) edge-on galaxies not suitable for simple automatic analysis because
these objects show signs of interaction, warps, or nearby bright stars project
on it. We added more candidate galaxies from RFGC, EFIGI, RC3, and Galaxy Zoo
catalogs found in the SDSS footprints. Our final sample consists of 5747
genuine edge-on galaxies. We estimate the structural parameters of the stellar
disks (the stellar disk thickness, radial scale length, and central surface
brightness) in the galaxies by analyzing photometric profiles in each of the g,
r, and i images. We also perform simplified 3-D modeling of the light
distribution in the stellar disks of edge-on galaxies from our sample. Our
large sample is intended to be used for studying scaling relations in the
stellar disks and bulges and for estimating parameters of the thick disks in
different types of galaxies via the image stacking. In this paper we present
the sample selection procedure and general description of the sample.Comment: Accepted for publication in Ap
Near-infrared and optical observations of galactic warps: A common, unexplained feature of most discs
Context: Warps occurring in galactic discs have been studied extensively in
HI and in the optical, but rarely in the near-infrared (NIR) bands that trace
the older stellar populations.
Aims: We provide NIR data of nearby edge-on galaxies, combined with optical
observations, for direct comparison of the properties of galactic warps as a
function of wavelength, and calculate warp curves for each galaxy and obtain
the characteristic warp parameters. We discuss these properties as possible
constraints to the different mechanisms that have been proposed for the
development and persistence of galactic warps.
Methods: We observed 20 galaxies that were selected from a statistically
complete diameter-limited subsample of edge-on disc galaxies. We used the Cerro
Tololo Infrared Imager (CIRIM) at the CTIO 1.5m Ritchey-Chretien telescope to
acquire the NIR data. We used the 1.54m Danish and 0.92m Dutch telescopes at
the European Southern Observatory's La Silla site for our optical observations.
Results: Our results show that 13 of our 20 sample galaxies are warped, with
the warp more pronounced in the optical than at NIR wavelengths. In the
remaining seven galaxies, no warp is apparent within the limitations of our
automated detection method. The transition between the unperturbed inner disc
and the outer, warped region is rather abrupt. S0 galaxies exhibit very small
or no warps. The magnetic model remains one of a number of interesting
formation scenarios.Comment: 16 page
Formation of a polar-ring galaxy in a galaxy merger
We numerically investigate stellar and gas dynamics in star-forming and
dissipative galaxy mergers between two disk galaxies with specific orbital
configurations. We find that violent relaxation combined with gaseous
dissipation in galaxy merging transforms two disk galaxies into one S0 galaxy
with polar-rings: Both the central S0-like host and the polar-ring component in
a polar-ring galaxy are originally disk galaxies. We also find that morphology
of the developed polar-rings reflects both the initial orbit configuration of
galaxy merging and the initial mass ratio of the two merger progenitor disk
galaxies. Based upon these results, we discuss the origin of the fundamental
observational properties of polar-ring galaxies, such as the prevalence of S0
galaxies among polar-ring galaxies, the rarity of polar-ring galaxies among S0
galaxies, the dichotomy between narrow polar-rings and annular ones, shapes of
polar-ring warps, and an appreciably larger amount of interstellar gas in the
polar-ring component.Comment: 42 pages, 19 figures, ApJ in pres
Chemical abundances in the polar disk of NGC4650A: implications for cold accretion scenario
The aim of the present study is to test whether the cold accretion of gas
through a "cosmic filament" Macci\`o et al. 2006 is a possible formation
scenario for the polar disk galaxy NGC 4650A. If polar disks form from cold
accretion of gas, the abundances of the HII regions may be similar to those of
very late-type spiral galaxies, regardless of the presence of a bright central
stellar spheroid, with total luminosity of few 10^9 Lsun. We use deep long slit
spectra obtained with the FORS2 spectrograph at the VLT in the optical and
near-infrared wavelength ranges for the brightest HII regions in the disk polar
disk of NGC 4650A. The strongest emission lines ([OII] Hbeta, [OIII], Halpha)
were used to derived oxygen abundances, metallicities and the global star
formation rates for the disk. The deep spectra available allowed us to measure
the Oxygen abundances (12 + log (O/H)) using the "Empirical method" based on
intensities of the strongest emission lines, and the "Direct method", based on
the determination of the electron temperature from the detection of weak
auroral lines, as the [OIII] at 4363 Angstrom. The Oxygen abundance measured
for the polar disk is then compared with those measured for different galaxy
types of similar total luminosities, and then compared against the predictions
of different polar ring formation scenarios. The average metallicity values for
the polar disk in NGC 4650A is Z=0.2 Zsun, and it is lower that the values
measured for ordinary spirals of similar luminosity. Moreover the gradient of
the metallicity is flat along the polar disk major axis, which implies none or
negligible metal enrichment from the stars in the older central spheroid. The
low metallicity value in the polar disk NGC 4650A and the flat metallicity
gradient are both consistent with a later infall of metal-poor gas, as expected
in the cold accretion processes.Comment: 42 pages, 9 figures. Accepted for publication in Ap
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